1. Field of the Invention
This invention relates to the chemical analysis of substances, and more particularly to a method and apparatus for the precise metering of biological fluids onto test elements.
2. State of the Prior Art
A number of automated systems have been developed for performing quantitative chemical analyses of fluid samples. Most of the commercially-available systems utilize liquid reagents and require analyzer equipment having intricate solution handling and transport capabilities. Recent developments, however, have provided test elements in essentially planar, dry form which can be loaded into a cartridge for use in an analyzer. In the use of such an analyzer, a test element from a cartridge is fed into a metering station where a predetermined amount of sample fluid is deposited on the test element. After an incubation period, the element is moved to a read station where a change in the test element is measured, the amount of change being proportional to a particular analyte in the fluid. The test element is used only once and is discarded after the reading has been taken. An analyzer for use with such test elements is disclosed in commonly-assigned U.S. Pat. No. 4,152,390.
Test elements of the type described above are adapted to function with very small quantities of sample fluid. For example, test elements for performing colorimetric analyses can produce a measurable response with only 10 microliters of sample fluid, and elements for performing potentiometric analyses are operable with 10 microliters of sample fluid and 10 microliters of reference fluid. The volume of fluid supplied to the elements should preferably not vary more than 5% from a selected value to achieve desirable test results. Thus, there is a problem in providing a metering device which can deliver precise micro quantities of fluid, in spite of variations in the physical properties of the fluid and the test elements. Moreover, in high-throughput analyzers, the metering device must be capable of repeatedly and accurately dispensing such quantities of fluid onto the test elements as they are sequentially moved into a metering station.
A metering device for use with planar test elements is shown in commonly-owned U.S. Pat. No. 4,142,656, to Smith et al. In this patent, fluid is dispensed from a sample cup having a dispensing tip formed on a bottom wall thereof. An electrically-actuated pump is used to generate a pressure in the cup sufficiently above ambient to form a pendant drop on the dispensing tip. The test element is then moved into contact with the pendant drop to effect a transfer of the fluid to the element. Both the sample cup and the test element are transported to the metering apparatus. The metering device disclosed in the Smith et al. patent requires complex transport and drive elements for both the sample cup and the test element, and it is not intended for use in applications where fluid must be aspirated into the metering device.
The patent to Drozdowski et al., U.S. Pat. No. 3,832,135, discloses a metering device which is adapted to pick up a disposable tip, aspirate fluid into the tip, meter fluid into a receptacle, and eject the tip. Drozdowski et al., however, do not contemplate the metering of fluids onto a series of analysis slides. The patent to Lightner, U.S. Pat. No. 3,988,921, shows apparatus for metering through a capillary tube onto a chromatographic plate. In the Lightner apparatus, the capillary tube is loaded with a quantity of fluid, and all of the fluid in the tube is dispensed onto a single plate. Since the tube must be reloaded for each new plate, the Lightner apparatus would not be suitable for use in a high-throughput analyzer where a series of tests are performed on a single sample fluid. Further, none of the known prior-art devices is directed to solving the problem of aspirating a fluid from a sample cup and depositing predetermined amounts of the fluid onto a plurality of analysis slides sequentially moved into a metering station.